The present invention relates generally to a superconducting apparatus such as a superconducting turbine generator or the like and more particularly to a thermal radiation shield for use in such an apparatus.
Today, superconducting technology is being developed to provide practical applications for the superconducting properties of certain metals at low temperatures, specifically at temperatures within the liquid helium region. For example, the desirability of an electrical generator rotor with a superconducting field winding has led to many design studies. One viable configuration which has heretofore been suggested includes a supercooled inner rotor, specifically one cooled to a temperature of approximately -450.degree. F., containing a super-conducting field winding and a concentric warmer (approximately +68.degree. F. or higher) outer rotor which serves as the main structural element of the overall rotor assembly. A thermal radiation shield is utilized for thermally insulating the inner rotor. This particular configuration does of course include other components as will be described hereinafter.
One important aspect of a superconducting apparatus of the type generally recited is to keep the total heat leakage into its cold zone, that is, in the area around its supercooled inner rotor, relatively low in order to minimize the amount of refrigerant, specifically helium, required to supercool the inner rotor. This is very important if the superconducting generator is to compete with conventional generators, both in terms of economy and efficiency. There are a number of effective ways to reduce this total leakage, which leakage may result from thermal conduction, thermal convention, and thermal radiation. One way is to minimize the conduction paths between the cold zone and the warm zone, that is, the zone outside the supercooled inner rotor, by supporting the cold zone to the warm zone utilizing essentially only two long, thin torque tubes and associated torque tube heat exchangers. Moreover, heat transfer by means of both conduction and convection, specifically by ambient gases, are minimized by providing a high vacuum environment around the cold zone. However, in order to reduce the radiant transfer of heat into the cold zone, the most promising suggestion has been to utilize a cylindrical radiation shield in the annulus between the inner and outer rotors. This shield functions by intercepting thermal radiation which would otherwise impinge upon the surface of the cold zone.
There are a number of inherent problems associated with the utilization of a radiation shield of the type just described. In the first place, it should be apparent that the shield which is exposed to the warm zone will be of a higher temperature than the cold zone. Applicants have found that to increase efficiency, the shield should itself be cooled and, in accordance with the present invention, to maximize efficiency the shield should be cooled, preferably internally, by the direct application of a fluid refrigerant, specifically helium gas. Applicants have found that this in turn results in certain inherent problems. These problems include the difficulty of manufacturing a reliable, structurally sound and economical shield and yet one which is contemplated for use in a severe environment such as that of a superconducting turbine generator or the like. The shield is to be placed in the annulus between the inner rotor and outer rotor, which annulus is to be minimized for electrical coupling reasons and yet this creates additional problems. Specifically, this conflicts with the need to have a mechanically sound, relatively thick shield which is to survive nonuniform short circuit crushing loads, centrifugal overspeed stresses, short circuit torques, and internal helium pressures in the event of a winding quench or normalization. Moreover, radial deflections of the shield must be limited so as not to interfere with adjacent machine parts.
As will be seen hereinafter, the present invention provides a radiation shield which is cooled, preferably internally cooled, by the direct application of a fluid refrigerant and yet an economical shield which is structurally sound, reliable in use and readily assembled.